Expose provider and model to AI agents

[ocots]

Summary

• Add the resolved provider and model to the system message sent to the agent.
• Preserve the agent instructions while appending a Runtime context section.
• Keep provider and model values parameterized and secret-free.

Agents can now reliably identify the provider and model used for their invocation, as requested by the generic hello agent.

Related PR

• CTBase caller already points to CTActions@main: Use merged AI agent workflow and Handbook CTBase.jl#560

Test plan

• Validate the workflow as YAML.
• Run git diff --check.
• Verify the generated JSON payload locally with jq.
• Merge and re-run the CTBase AI Hello workflow.